Men with low testosterone levels that contract COVID-19 are at far greater risk of dying from the virus, a study has found.
A recent study found men are twice as likely to die from the coronavirus, but clinicians have been unable to determine why this is.
But a study from a German hospital of 45 COVID-19 patients admitted to intensive care reveals the male sex hormone testosterone may play a key role.
The hormone is known to help regulate the body's immune response but when a man has low levels of testosterone, the immune system is not kept in check and can go haywire following infection.
This leads to a so-called cytokine storm which happens when the immune system goes out of control as it tries to kill the pathogen.
A cytokine storm eventually begins damaging the body itself and, if left unchecked, can be fatal.
The researchers assessed the first 45 laboratory-confirmed COVID-19 patients admitted to the ICU at the University Medical Center Hamburg-Eppendorf.
Thirty-five were men and ten were women, with seven patients requiring oxygen and 33 of them needing ventilation. Nine men and three women died.
Hormone levels of each patient were assessed on their first day in ICU, before they had received any invasive procedures.
Samples from the COVID-19 patients were tested for 12 hormones, including testosterone and dihydrotestosterone.
Testosterone is key in how the body initiates and regulates various immune responses, including fighting viral infections.
It plays a particular important role in men's immunity as it is the main male sex hormone.
Of the male COVID-19 patients sent to ICU at the German hospital, more than two thirds (68.6 per cent) recorded low levels of testosterone.
In contrast, the majority of female patients (60 per cent) had elevated testosterone levels.
While low levels of testosterone can not control the immune response in men, the study found that in female COVID-19 patients, higher testosterone levels were linked to a more significant inflammatory response.
Professor Gülsah Gabriel from the Leibniz Institute for Experimental Virology in Hamburg, who was involved in the research, told MailOnline: 'The majority of male COVID-19 patients had low testosterone levels.
'Of those male COVID-19 patients who died, the majority also had low testosterone levels.
'Thus, low testosterone levels in men seem to be a risk factor for severe and even fatal disease outcome in men upon infection with so-called “cytokine inducing” respiratory viruses.'